The effect of washover geometry on sediment transport during inundation events

Storm-induced sediment transport across a barrier island can lead to vertical accretion and onshore migration of the barrier island. Many barrier islands either have high dunes that prevent inundation, or are so low-lying that they are inundated several times a year. The Wadden Islands in the Netherlands, Germany and Denmark typically have alongshore-varying topography, where high dunes alternate with low-lying washover openings. The effects of the geometry of the washover openings on hydrodynamics and sediment transport are still unknown and are the main focus of this research. First, we present data on width and for some cases also vertical elevation of bed level for all washover openings along the Wadden Islands. The mean width is 200 m but the actual width ranges from 35 to 1100 m, and the elevation is between 1.5 and 2.1 m above MSL. Further, we present results of an XBeach model study to investigate how the washover opening geometry affects sediment transport during storm-induced inundation. We identify two important effects of washover width: firstly, for narrow openings flow contraction is important, causing relatively larger sediment exchange rates per unit width; secondly, in a wider opening sediment is transported over a larger width, resulting in larger sediment mass exchange rates. Furthermore, the elevation of the washover opening is of high importance: washover openings that are 30 cm higher than the reference case significantly decrease currents and sediment transport across the island. Divergence of sediment transport occurs in the washover opening, which leads to erosional patterns. Landward from the opening, sediment transport converges which leads to depositional patterns. The pressure gradient between North Sea and Wadden Sea across the Wadden Islands is an important forcing parameter: higher water levels in the back-barrier reduce onshore-directed currents and sediment transport.